Optical modulator

Abstract

To provide an optical modulator that can suppress the optical loss and increase the modulation frequency.SOLUTION: An optical modulator (100) includes an optical waveguide (2), a first electrode (3), a second electrode (4), and a low dielectric constant layer (5). The optical waveguide (2) is formed of a material with an electro-optic effect. The first electrode (3) and the second electrode (4) form a potential difference from each other. In a cross-sectional view perpendicular to a direction where the optical waveguide (2) extends, the first electrode (3) is provided on one side in the width direction of the optical waveguide (2) and on one side in the thickness direction of the optical waveguide (2), the second electrode (4) is provided on the other side in the width direction of the optical waveguide (2) and on the other side in the thickness direction of the optical waveguide (2), the low dielectric constant layer (5) exists between the first electrode (3) and the optical waveguide (2), and a part of the first electrode (3) that is close to the optical waveguide (2) is embedded in the low dielectric layer (5).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to optical modulators. [Background technology]

[0002] The spread of mobile devices and cloud computing has led to a dramatic increase in internet traffic. This has led to an expansion in demand for optical communications. Optical communications require optical transceivers to convert optical signals into electrical signals and vice versa. An optical transceiver has an optical modulator as its main component. The optical modulator converts electrical signals into optical signals.

[0003] A conventional optical modulator is disclosed, for example, in Japanese Patent Laid-Open No. 2008-250080 (Patent Document 1). The optical modulator in Patent Document 1 includes a thin plate having an electro-optic effect, an optical waveguide formed in the thin plate, and a control electrode for controlling light passing through the optical waveguide. The control electrode includes a first electrode and a second electrode, which are arranged to sandwich the thin plate. The first electrode includes a coplanar electrode including at least a signal electrode and a ground electrode. The second electrode includes at least a ground electrode. A low-refractive index layer having a width greater than that of at least the signal electrode of the first electrode is formed below the thin plate. In some cases, a buffer layer is formed at least between the thin plate and the first electrode. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-250080 Summary of the Invention [Problem to be solved by the invention]

[0005] In the optical modulator of Patent Document 1, the signal electrode of the first electrode has a rectangular shape in a cross section perpendicular to the extension direction of the optical waveguide, and this signal electrode is aligned with the optical waveguide in the thickness direction of the optical waveguide. In this case, the entire bottom surface of the signal electrode faces the optical waveguide, so light leaking from the optical waveguide is easily absorbed by the signal electrode, resulting in optical loss. Furthermore, in the optical modulator of Patent Document 1, a buffer layer is formed between the first electrode including the signal electrode and the thin plate. The buffer layer contributes to adjusting the effective refractive index. Without the buffer layer, the difference between the effective refractive index perceived by the electrical signal and the effective refractive index perceived by the light wave would not be reduced, making it impossible to increase the modulation frequency.

[0006] The present disclosure has been made in view of the above-mentioned problems, and an object of the present disclosure is to provide an optical modulator that can suppress optical loss and increase the modulation frequency. [Means for solving the problem]

[0007] The optical modulator according to the present disclosure includes an optical waveguide made of a material having an electro-optic effect, a control electrode for controlling light passing through the optical waveguide, and a low-dielectric-constant layer having a dielectric constant lower than that of the optical waveguide. The control electrode includes a first electrode and a second electrode that generate a potential difference between them. In a cross-sectional view perpendicular to the direction in which the optical waveguide extends, the first electrode is provided on one side of the optical waveguide in the width direction and one side of the optical waveguide in the thickness direction, and the second electrode is provided on the other side of the optical waveguide in the width direction and the other side of the optical waveguide in the thickness direction. In this cross-sectional view, a low-dielectric-constant layer is interposed between the first electrode and the optical waveguide, and a portion of the first electrode close to the optical waveguide is embedded in the low-dielectric-constant layer. [Effects of the Invention]

[0008] According to the optical modulator according to the present disclosure, it is possible to suppress optical loss and increase the modulation frequency. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic diagram showing a cross section of an optical modulator according to the first embodiment. [Figure 2] FIG. 2 is a schematic diagram for explaining the properties of the substrate in the optical modulator according to the first embodiment. [Figure 3] FIG. 3 is a schematic diagram showing a cross section of the optical modulator according to the second embodiment. [Figure 4] FIG. 4 is a schematic diagram showing a cross section of an optical modulator according to the third embodiment. [Figure 5] FIG. 5 is a schematic diagram showing a cross section of an optical modulator according to the fourth embodiment. [Figure 6] FIG. 6 is a schematic diagram showing a cross section of an optical modulator according to a fifth embodiment. [Figure 7] FIG. 7 is a schematic diagram showing a cross section of an optical modulator according to the sixth embodiment. [Figure 8] FIG. 8 is a schematic diagram showing a cross section of an optical modulator according to the seventh embodiment. [Figure 9] FIG. 9 is a schematic diagram showing a cross section of an optical modulator according to the eighth embodiment. [Figure 10] FIG. 10 is a schematic diagram showing a cross section of an optical modulator according to the ninth embodiment. [Figure 11] FIG. 11 is a schematic diagram showing a cross section of an optical modulator according to the tenth embodiment. [Figure 12] FIG. 12 is a schematic diagram showing a cross section of an optical modulator according to the eleventh embodiment. [Figure 13A] FIG. 13A is a schematic diagram showing the strength of the electric field when the first electrode and the second electrode are not stretched. [Figure 13B] FIG. 13B is a schematic diagram showing the strength of the electric field when only the first electrode is stretched. [Figure 13C] FIG. 13C is a schematic diagram showing the strength of the electric field when both the first electrode and the second electrode are stretched. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present disclosure will be described. Note that in the following description, examples of embodiments of the present disclosure will be described, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and specific materials may be exemplified, but the present disclosure is not limited to these examples.

[0011] The optical modulator according to this embodiment includes an optical waveguide made of a material having an electro-optic effect, a control electrode for controlling light passing through the optical waveguide, and a low-dielectric-constant layer having a dielectric constant lower than that of the optical waveguide. The control electrode includes a first electrode and a second electrode that generate a potential difference between them. In a cross-sectional view perpendicular to the direction in which the optical waveguide extends, the first electrode is provided on one side of the optical waveguide in the width direction and one side of the optical waveguide in the thickness direction, and the second electrode is provided on the other side of the optical waveguide in the width direction and the other side of the optical waveguide in the thickness direction. In this cross-sectional view, a low-dielectric-constant layer is interposed between the first electrode and the optical waveguide, and a portion of the first electrode close to the optical waveguide is embedded in the low-dielectric-constant layer (first configuration).

[0012] In the optical modulator of the first configuration, the first electrode is offset to one of the two sides in the width direction of the optical waveguide with respect to the optical waveguide, and the second electrode is offset to the other of the two sides in the width direction of the optical waveguide. Furthermore, the first electrode is offset to one of the two sides in the thickness direction of the optical waveguide with respect to the optical waveguide, and the second electrode is offset to the other of the two sides in the thickness direction of the optical waveguide. That is, the first electrode, optical waveguide, and second electrode are arranged diagonally in this order with respect to the width and thickness directions of the optical waveguide. Furthermore, a low-dielectric layer is interposed between the first electrode and the optical waveguide, and a portion of the first electrode close to the optical waveguide is embedded in this low-dielectric layer. The first electrode is not in contact with the optical waveguide.

[0013] By shifting the positions of the first and second electrodes in the thickness and width directions relative to the optical waveguide, the opposing areas of the first and second electrodes and the optical waveguide are smaller than when the rectangular electrodes are aligned with the optical waveguide in the thickness or width direction of the optical waveguide and the entire surface of the electrodes faces the optical waveguide. This reduces the likelihood of light leaking from the optical waveguide being absorbed by the first and second electrodes, thereby reducing optical loss.

[0014] Furthermore, the electric field from the first electrode toward the optical waveguide passes through the low-dielectric layer. This reduces the effective refractive index felt by the electric signal. At this time, since at least a portion of the first electrode close to the optical waveguide is buried in the low-dielectric layer, the contact area of ​​the first electrode with the low-dielectric layer is larger than when the first electrode is simply placed on the low-dielectric layer, and therefore a larger electric field passes through the low-dielectric layer. This allows the effective refractive index felt by the electric signal to be smaller than usual. Usually, the effective refractive index felt by the electric signal is larger than the effective refractive index felt by the light wave. This reduces the difference between the effective refractive index felt by the electric signal and the effective refractive index felt by the light wave. Therefore, the modulation frequency can be increased.

[0015] In the optical modulator of the first configuration, the first electrode may include a corner. This corner is disposed on the optical waveguide side and is embedded in the low-dielectric layer (second configuration). In this case, the electric field is concentrated at the corner of the first electrode, thereby increasing the strength of the electric field from the first electrode toward the optical waveguide. This makes it possible to suppress a decrease in the electric field applied to the optical waveguide between the first electrode and the second electrode.

[0016] The optical modulator of the first or second configuration has, for example, the following configuration: In a cross-sectional view perpendicular to the direction in which the optical waveguide extends, the optical waveguide includes a first side extending in the width direction and a second side disposed parallel to the first side and extending in the width direction. In this cross-sectional view, the first electrode is provided on the first side (third configuration).

[0017] The optical modulator of the first or second configuration may have the following configuration: In a cross section perpendicular to the direction in which the optical waveguide extends, the optical waveguide has a semi-elliptical shape including a base as a major axis extending in the width direction. In this cross section, the first electrode is provided on the base side (fourth configuration).

[0018] The optical modulator may further include the following configuration: In a cross-sectional view perpendicular to the extending direction of the optical waveguide, the first electrode has a rectangular shape, and the first electrode is embedded in the surface of the low dielectric layer opposite to the optical waveguide (fifth configuration).

[0019] The optical modulator may further include the following configuration: In a cross-sectional view perpendicular to the extending direction of the optical waveguide, the first electrode is provided only on one side of the center in the thickness direction of the optical waveguide, and the second electrode is provided only on the other side of the center in the thickness direction of the optical waveguide (sixth configuration).

[0020] The optical modulator preferably has the following configuration: An auxiliary low dielectric layer is provided between the second electrode and the optical waveguide, and the auxiliary low dielectric layer has a lower dielectric constant than the optical waveguide (seventh configuration).

[0021] In the optical modulator of the seventh configuration, an auxiliary low-dielectric layer is provided between the second electrode and the optical waveguide, so the electric field also passes through the auxiliary low-dielectric layer. This further reduces the effective refractive index felt by the electrical signal. This further reduces the difference between the effective refractive index felt by the electrical signal and the effective refractive index felt by the optical wave. This allows for a higher modulation frequency.

[0022] The optical modulator preferably has the following configuration. The material of the optical waveguide is LiNbO3 (eighth configuration). LiNbO3 (lithium niobate) has a particularly high electro-optic effect. In this specification, LiNbO3 may be referred to as LN. The material of the optical waveguide is not particularly limited as long as it has an electro-optic effect. For example, the material of the optical waveguide may be LiTaO3 (lithium tantalate), PLZT (lead lanthanum zirconate titanate), KTN (potassium tantalate niobate), BaTiO3 (barium titanate), or the like.

[0023] The optical modulator preferably has the following configuration: One of the first electrode and the second electrode extends from the optical waveguide in the thickness direction, and the other extends from the optical waveguide in the width direction (ninth configuration). In this case, the electric field with respect to the optical waveguide becomes stronger.

[0024] The optical modulator may further include a substrate on which an optical waveguide is provided (tenth configuration).

[0025] The optical modulator of the tenth configuration may have the following configuration. The substrate is made of the same material as the optical waveguide, and the optical waveguide is ridge-shaped (eleventh configuration). In this case, it is possible to cover the periphery of the optical waveguide except for the boundary with the substrate with a low-dielectric layer. This makes it easy to adjust the effective refractive index. Furthermore, it is possible to further confine light within the optical waveguide.

[0026] However, optical waveguides can also be formed by diffusing titanium (Ti) into the substrate. Optical waveguides can also be formed by proton exchange.

[0027] The optical modulator of any one of the first to ninth configurations may include two optical modulator units arranged in parallel, each including an optical waveguide, a control electrode, and a low-dielectric layer (twelfth configuration).

[0028] The optical modulator of the twelfth configuration is a Mach-Zehnder type optical modulator. In this case, intensity modulation is possible in addition to phase modulation. This allows multi-level modulation to be performed, and the transmission capacity can be increased. Moreover, the optical modulator of the twelfth configuration has the same effects as the first to ninth configurations.

[0029] The optical modulator of the twelfth configuration may have the following configuration: Of the two optical modulator units, the first electrode of one optical modulator unit is formed integrally with the first electrode of the other optical modulator unit (thirteenth configuration). In this case, the first electrode of one optical modulator unit can be shared with the first electrode of the other optical modulator unit.

[0030] The optical modulator of the twelfth or thirteenth configuration may have the following configuration: Each of the optical modulator units further includes a substrate on which an optical waveguide is provided, and the substrate of one of the two optical modulator units is arranged in parallel with the substrate of the other optical modulator unit (fourteenth configuration).

[0031] The optical modulator of the fourteenth configuration may have the following configuration. In each of the optical modulator units, the substrate is made of the same material as the optical waveguide, and the optical waveguide is of a ridge type (fifteenth configuration). The optical modulator of the fifteenth configuration corresponds to the eleventh configuration. Therefore, as with the eleventh configuration, it is easy to adjust the effective refractive index, and furthermore, it is possible to better confine light within the optical waveguide.

[0032] The optical modulator of the fourteenth or fifteenth configuration may have the following configuration: Of the two optical modulator units, the substrate of one optical modulator unit is integrated with the substrate of the other optical modulator unit, and the optical waveguide of one optical modulator unit and the optical waveguide of the other optical modulator unit have mutually opposite spontaneous polarization directions. Voltages of the same phase are applied to the first electrode of one optical modulator unit and the first electrode of the other optical modulator unit (sixteenth configuration).

[0033] In the optical modulator of the sixteenth configuration, the substrate of one optical modulator unit can be shared with the substrate of the other optical modulator unit. The optical waveguides of one optical modulator unit and the other optical modulator unit are provided on a shared substrate. This allows the distance between the optical waveguide of one optical modulator unit and the optical waveguide of the other optical modulator unit to be reduced. In this case, the overall width of the optical modulator can be narrowed.

[0034] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In each drawing, the same or equivalent components are designated by the same reference numerals, and the same description will not be repeated.

[0035] First Embodiment [Configuration of optical modulator 100] FIG. 1 is a schematic diagram showing a cross section of an optical modulator 100 according to the first embodiment. FIG. 1 shows a cross section perpendicular to the extension direction of an optical waveguide 2. The extension direction of the optical waveguide 2 can also be referred to as the direction along the optical waveguide 2. In this specification, unless otherwise specified, a cross section refers to a cross section perpendicular to the extension direction of the optical waveguide 2 or optical waveguides 2A and 2B described below. In the cross section of the optical modulator 100, a support plate 7 that supports the entire device is at the bottom, the thickness direction of the optical modulator 100 corresponds to the up-down direction, and the width direction of the optical modulator 100 corresponds to the left-right direction. However, in this specification, the terms up, down, left, and right are defined for the convenience of explanation and do not limit the actual position of the optical modulator 100.

[0036] 1, the optical modulator 100 includes a substrate 1, an optical waveguide 2, a first electrode 3, a second electrode 4, and a low dielectric layer 5. The first electrode 3 and the second electrode 4 are included in control electrodes for controlling light passing through the optical waveguide 2.

[0037] The first electrode 3 and the second electrode 4 form a potential difference with each other. The first electrode 3 is, for example, a signal electrode. The second electrode 4 is not particularly limited as long as it forms a potential difference with the first electrode 3. The second electrode 4 is, for example, a ground electrode. The second electrode 4 may be an inverse signal electrode that applies a voltage of opposite phase to the potential of the first electrode 3.

[0038] The second electrode 4 is disposed at a position lower than the first electrode 3. The substrate 1, the optical waveguide 2, the low dielectric layer 5, the first electrode 3, and the second electrode 4 are supported by a support plate 7. The support plate 7 is disposed at the bottom.

[0039] The optical waveguide 2 is made of a material that has an electro-optic effect. The material of the optical waveguide 2 is, for example, LN. The optical waveguide 2 is formed on the substrate 1. Specifically, the optical waveguide 2 is formed on the top of the substrate 1. This optical waveguide 2 is formed by diffusing Ti into the substrate 1. The portion of the substrate 1 where Ti is diffused has a high refractive index and can confine light, so it can be used as the optical waveguide 2.

[0040] The optical waveguide 2 may have a cross-sectional shape in which the width (horizontal dimension) is greater than the thickness (vertical dimension). In FIG. 1, the cross-sectional shape of the optical waveguide 2 is substantially a wide, roughly rectangular shape. In this case, the cross-sectional shape of the optical waveguide 2 includes a first side extending in the width direction and a second side arranged parallel to the first side and extending in the width direction. The cross-sectional shape of the optical waveguide 2 further includes a third side and a fourth side each extending in the thickness direction. In the example shown in FIG. 1, the first side and the second side are a pair of long sides, and the third side and the fourth side are a pair of short sides. When the cross-sectional shape of the optical waveguide 2 is a wide rectangle, one of the pair of long sides (the upper first side) is on the surface of the substrate 1, and the other long side (the lower second side) is inside the substrate 1.

[0041] In the cross section of the optical waveguide 2, the first and second long sides are connected by the third and fourth short sides. In the example shown in FIG. 1 , the third and fourth sides of the optical waveguide 2 are linear in the cross section of the optical modulator 100 and are parallel to the thickness direction of the optical waveguide 2. However, the third and fourth sides may be inclined with respect to the thickness direction of the optical waveguide 2 and are not necessarily linear. In the cross section of the optical modulator 100, the third and fourth sides of the optical waveguide 2 may be curved or may have a shape that combines straight and curved lines. Furthermore, the length of the third side may be the same as or different from the length of the fourth side. Similarly, the length of the first side may be the same as or different from the length of the second side.

[0042] The cross-sectional shape of the optical waveguide 2 may be a wide semi-ellipse. In this case, the cross-sectional shape of the optical waveguide 2 includes a base as a major axis extending in the width direction and elliptical arc-shaped sides extending in the width direction. When the cross-sectional shape of the optical waveguide 2 is a wide semi-ellipse, the base is on the surface of the substrate 1 and the elliptical arc-shaped sides are inside the substrate 1.

[0043] A low-dielectric layer 5 is laminated on the substrate 1. Therefore, the low-dielectric layer 5 is laminated on the optical waveguide 2. In this case, the low-dielectric layer 5 directly covers the upper surface of the optical waveguide 2 and the upper surface of the substrate 1 around it. For example, when the cross-sectional shape of the optical waveguide 2 is a wide rectangle, the low-dielectric layer 5 is provided mainly along one of the long sides (upper long side) of the optical waveguide 2 in the cross section of the optical modulator 100. When the cross-sectional shape of the optical waveguide 2 is a horizontally elongated semi-ellipse, the low-dielectric layer 5 is provided mainly along the bottom side of the optical waveguide 2 in the cross section of the optical modulator 100. The dielectric constant of the low-dielectric layer 5 is lower than that of the optical waveguide 2. The material of the low-dielectric layer 5 is not particularly limited as long as its dielectric constant is lower than that of the optical waveguide 2. An oxide (e.g., Al2O3, SiO2, LaAlO3, LaYO3, ZnO, HfO2, MgO, or Y2O3) is used as the low-dielectric layer 5. A polymer (e.g., BCB (benzocyclobutene), PI (polyimide)) may also be used as the low-dielectric layer 5.

[0044] The first electrode 3 is disposed above the substrate 1. The second electrode 4 is disposed below the substrate 1. From another perspective, the second electrode 4 is buried below the substrate 1. The first electrode 3 and the second electrode 4 are made of a metal material and each have a rectangular cross-sectional shape. For example, if the cross-sectional shape of the optical waveguide 2 is a wide rectangle, the first electrode 3 has a pair of sides parallel to the one long side (the upper first side) of the optical waveguide 2 in the cross section of the optical modulator 100. If the cross-sectional shape of the optical waveguide 2 is a wide semi-ellipse, the first electrode 3 has a pair of sides parallel to the bottom side of the optical waveguide 2 in the cross section of the optical modulator 100. The first electrode 3 is buried in the surface of the low dielectric layer 5 opposite to the optical waveguide 2. In the present embodiment, when a portion of the first electrode 3 is embedded in the low dielectric layer 5, the thickness (vertical dimension) of the low dielectric layer 5 in the portion where the first electrode 3 is located is significantly smaller than the thickness of the low dielectric layer 5 in other portions.

[0045] The first electrode 3 can be embedded in the low dielectric layer 5, for example, as follows. That is, first, the low dielectric layer 5 is formed on the surface of the substrate 1 on which the optical waveguide 2 is formed. Next, a groove is formed in the low dielectric layer 5 by photolithography and etching. After that, the first electrode 3 is formed by vapor deposition and lift-off in the groove. In this way, the first electrode 3 embedded in the low dielectric layer 5 can be formed.

[0046] Here, the first electrode 3 and the second electrode 4 are arranged to sandwich the optical waveguide 2 in a direction oblique to the thickness direction of the optical waveguide 2. The first electrode 3 is provided on one side of the optical waveguide 2 in the width direction and one side of the optical waveguide 2 in the thickness direction. The second electrode 4 is provided on the other side of the optical waveguide 2 in the width direction and the other side of the optical waveguide 2 in the thickness direction. In other words, with respect to the optical waveguide 2, the first electrode 3 is offset to one of the two sides of the optical waveguide 2 in the width direction (right in FIG. 1), and the second electrode 4 is offset to the other of the two sides of the optical waveguide 2 in the width direction (left in FIG. 1). Furthermore, with respect to the optical waveguide 2, the first electrode 3 is offset to one of the two sides of the optical waveguide 2 in the thickness direction (upper in FIG. 1), and the second electrode 4 is offset to the other of the two sides of the optical waveguide 2 in the thickness direction (lower in FIG. 1).

[0047] A low-dielectric layer 5 is interposed between the first electrode 3 and the optical waveguide 2. The lower part of the first electrode 3 is buried in the low-dielectric layer 5. That is, the part of the first electrode 3 that is close to the optical waveguide 2 is buried in the low-dielectric layer 5. From another perspective, the corner of the first electrode 3 that is located on the optical waveguide 2 side is buried in the low-dielectric layer 5. In this case, the corner of the first electrode 3 is near the optical waveguide 2. The first electrode 3 is not in contact with the optical waveguide 2.

[0048] In this embodiment, in the cross section of the optical modulator 100, the first electrode 3 is provided only on one side of the center in the thickness direction of the optical waveguide 2, and the second electrode 4 is provided only on the other side of the center in the thickness direction of the optical waveguide 2. For example, when the cross section of the optical waveguide 2 is a wide rectangle, the first electrode 3 is provided entirely on one long side (upper long side) of the center in the thickness direction of the optical waveguide 2, and the second electrode 4 is provided entirely on the other long side (lower long side) of the center in the thickness direction of the optical waveguide 2. When the cross section of the optical waveguide 2 is a wide semi-ellipse, the first electrode 3 is provided entirely on the bottom side of the center in the thickness direction of the optical waveguide 2, and the second electrode 4 is provided entirely on the side of the elliptical arc of the optical waveguide 2.

[0049] In this embodiment, when viewed in the left-right direction, the first electrode 3 has no portion that overlaps with the optical waveguide 2. Similarly, when viewed in the left-right direction, the second electrode 4 has no portion that overlaps with the optical waveguide 2. Similarly, when viewed in the up-down direction, the first electrode 3 has no portion that overlaps with the optical waveguide 2. Similarly, when viewed in the up-down direction, the second electrode 4 has no portion that overlaps with the optical waveguide 2. A support plate 7 is laminated below the substrate 1.

[0050] [effect] According to the optical modulator 100 of this embodiment, when the optical modulator 100 is operating, an electric field acts from the first electrode 3 toward the second electrode 4, and an electric field is applied to the optical waveguide 2. At this time, in the optical modulator 100 of this embodiment, the first electrode 3 and the second electrode 4 are arranged to sandwich the optical waveguide 2 in a direction oblique to the thickness direction of the optical waveguide 2, and the portion of the first electrode 3 that is close to the optical waveguide 2 is embedded in the low dielectric layer 5. As a result, the following effects are obtained.

[0051] In the optical modulator 100 of this embodiment, compared to when the signal electrode is not embedded in the low-dielectric layer but simply contacts the surface of a low-dielectric layer having a certain thickness, the portion of the first electrode 3 that is close to the optical waveguide 2, more specifically, the corners of the first electrode 3, are embedded in the low-dielectric layer 5 and are located close to the optical waveguide 2. Because the electric field is concentrated at the corners of the first electrode 3, the strength of the electric field from the first electrode 3 to the optical waveguide 2 increases. As a result, the electric field applied to the optical waveguide 2 does not decrease. Therefore, the decrease in the electric field applied to the optical waveguide 2 can be suppressed.

[0052] Furthermore, since the low-dielectric layer 5 is interposed between the first electrode 3 and the optical waveguide 2, the first electrode 3 is not in contact with the optical waveguide 2, and only a portion (a corner) of the first electrode 3 is located near the optical waveguide 2. Therefore, even if the shortest distance between the optical waveguide 2 and the first electrode 3 is the same, the area of ​​the first electrode 3 facing the optical waveguide 2 is smaller in the optical modulator 100 of this embodiment than when the top surface of the optical waveguide having a rectangular cross section is arranged opposite the bottom surface of the signal electrode having a rectangular cross section. Therefore, absorption of light leaking from the optical waveguide 2 by the first electrode 3 is suppressed. Therefore, optical loss can be suppressed.

[0053] Furthermore, the electric field from the first electrode 3 toward the optical waveguide 2 passes through the low-dielectric layer 5. This reduces the effective refractive index felt by the electric signal. Since the portion of the first electrode 3 closest to the optical waveguide 2, more specifically the corners of the first electrode 3, are embedded in the low-dielectric layer 5, the contact area between the first electrode 3 and the low-dielectric layer 5 is larger than when the electrode is simply placed on the low-dielectric layer, and a larger electric field passes through the low-dielectric layer 5. This allows the effective refractive index felt by the electric signal to be smaller than usual. In general materials, the optical response includes ionic polarization, so the effective refractive index felt by an electric signal (GHz) is larger than the effective refractive index felt by an optical wave (THz). This reduces the difference between the effective refractive index felt by the electric signal and the effective refractive index felt by the optical wave. This allows for a higher modulation frequency.

[0054] [Cutting angle of substrate 1 material] Fig. 2 is a schematic diagram for explaining the properties of the optical waveguide 2 in the optical modulator 100 according to the first embodiment. Fig. 2 shows a cross section of the optical modulator 100. As shown in Fig. 2, the first electrode 3 and the second electrode 4 are arranged to sandwich the optical waveguide 2 in a direction oblique to the thickness direction of the optical waveguide 2.

[0055] When an electric field is applied to the optical waveguide 2 formed on the substrate 1, the refractive index changes due to the electro-optic effect. At this time, the direction of the electric field can be considered to be parallel to the line L (see the thick line in Figure 2) connecting the nearest corners of the first electrode 3 and the second electrode 4. If the tilt of the crystal axis (e.g., the c-axis in the case of LN) of the optical waveguide 2 is parallel to the direction of the electric field, the refractive index can be changed effectively. The tilt of the electric field, i.e., the tilt θ of the electrode arrangement, can be calculated based on the following equation (1) using the width direction (horizontal) component w and the thickness direction (vertical) component t of the shortest distance between the first electrode 3 and the second electrode 4. θ=arctan(t / w)×180 / π. (1)

[0056] If the tilt angle θ of the electrode arrangement is 0 to 5°, a wafer generally called an X-cut may be used as the material for the optical waveguide 2 (substrate 1).If the tilt angle θ of the electrode arrangement is 85 to 90°, a wafer generally called a Z-cut may be used as the material for the optical waveguide 2 (substrate 1).

[0057] Second Embodiment 3 is a schematic diagram showing a cross section of an optical modulator 100 according to the second embodiment. The optical modulator 100 of this embodiment is a modification of the optical modulator 100 of the first embodiment.

[0058] 3, the optical modulator 100 further includes an auxiliary low-dielectric-constant layer 6. Specifically, the auxiliary low-dielectric-constant layer 6 is laminated below the substrate 1. The dielectric constant of the auxiliary low-dielectric-constant layer 6 is lower than that of the optical waveguide 2, similar to that of the low-dielectric-constant layer 5. The material of the auxiliary low-dielectric-constant layer 6 is not particularly limited as long as the dielectric constant is lower than that of the optical waveguide 2. The material of the auxiliary low-dielectric-constant layer 6 may be the same as or different from the material of the low-dielectric-constant layer 5.

[0059] In this embodiment, the support plate 7 is laminated below the auxiliary low dielectric layer 6. Furthermore, the second electrode 4 is disposed inside the auxiliary low dielectric layer 6. That is, the auxiliary low dielectric layer 6 is provided between the second electrode 4 and the optical waveguide 2. In this case, the auxiliary low dielectric layer 6 directly covers the lower surface of the substrate 1, and also covers the lower surface of the optical waveguide 2.

[0060] In the optical modulator 100 of this embodiment, the auxiliary low dielectric layer 6 is provided between the second electrode 4 and the optical waveguide 2, so that the electric field also passes through the auxiliary low dielectric layer 6. This further reduces the effective refractive index felt by the electric signal. This further reduces the difference between the effective refractive index felt by the electric signal and the effective refractive index felt by the light wave. This allows the modulation frequency to be increased.

[0061] Third Embodiment 4 is a schematic diagram showing a cross section of an optical modulator 100 according to the third embodiment. The optical modulator 100 of this embodiment is a modification of the optical modulator 100 of the first embodiment.

[0062] Referring to FIG. 4, the substrate 1 has a ridge-type optical waveguide 2. That is, the substrate 1 has a convex rib on the top, which functions as the optical waveguide 2. The convex rib is formed on the substrate 1 by processing a wafer, which is the material. The convex rib can confine light in its thickness and width directions. The cross-sectional shape of the ridge-type optical waveguide 2 is roughly rectangular. Strictly speaking, the cross-sectional shape of the ridge-type optical waveguide 2 is often trapezoidal. In this embodiment, the first electrode 3 has a portion that very slightly overlaps with the optical waveguide 2 when viewed in the vertical direction. The second electrode 4 also has a portion that very slightly overlaps with the optical waveguide 2 when viewed in the vertical direction.

[0063] The substrate 1 is made of the same material as the optical waveguide 2. However, the material of the substrate 1 may be different from the material of the optical waveguide 2. In this case, the material of the substrate 1 is, for example, Si.

[0064] The optical modulator 100 of this embodiment achieves the same effects as the first embodiment. However, in this embodiment, since the optical waveguide 2 is of a ridge type, it is possible to cover the periphery of the optical waveguide 2 except for the boundary with the substrate 1 with the low-dielectric layer 5. In other words, the periphery of the optical waveguide 2 is widely covered with the low-dielectric layer 5. This makes it easy to adjust the effective refractive index. Furthermore, it is possible to further confine light within the optical waveguide 2.

[0065] The configuration of this embodiment may be applied to the optical modulator 100 of the second embodiment.

[0066] <Fourth embodiment> 5 is a schematic diagram showing a cross section of an optical modulator 100 according to a fourth embodiment. The optical modulator 100 of this embodiment is a modification of the optical modulator 100 of the third embodiment.

[0067] Referring to FIG. 5 , the optical waveguide 2 is of a ridge type. In this embodiment, the first electrode 3 has a portion that overlaps with the optical waveguide 2 when viewed in the vertical direction. The first electrode 3 may overlap within a range of 10% of the total width W from the end of the width direction within the width direction region of the optical waveguide 2. The second electrode 4 also has a portion that overlaps with the optical waveguide 2 when viewed in the vertical direction. The second electrode 4 may overlap within a range of 10% of the total width W from the end of the width direction within the width direction region of the optical waveguide 2.

[0068] The optical modulator 100 of this embodiment, like the third embodiment, has the same effects as the first embodiment. However, the configuration of this embodiment may be applied to an optical waveguide 2 formed in a substrate 1 by Ti diffusion.

[0069] Fifth Embodiment 6 is a schematic diagram showing a cross section of an optical modulator 100 according to a fifth embodiment. The optical modulator 100 of this embodiment is a modification of the optical modulator 100 of the third embodiment.

[0070] Referring to FIG. 6 , the optical waveguide 2 is of a ridge type. In this embodiment, the first electrode 3 does not overlap with the optical waveguide 2 when viewed in the vertical direction. However, the first electrode 3 overlaps with the optical waveguide 2 when viewed in the horizontal direction. The first electrode 3 may overlap with the optical waveguide 2 within a region in the thickness direction of the optical waveguide 2, within 10% of the total thickness T from the upper end. The second electrode 4 also does not overlap with the optical waveguide 2 when viewed in the vertical direction. However, the second electrode 4 overlaps with the optical waveguide 2 within a region in the thickness direction of the optical waveguide 2, within 10% of the total thickness T from the lower end. The second electrode 4 may overlap with the optical waveguide 2 within a region in the thickness direction of the optical waveguide 2, within 10% of the total thickness T from the lower end.

[0071] The optical modulator 100 of this embodiment, like the third embodiment, has the same effects as the first embodiment. However, the configuration of this embodiment may be applied to an optical waveguide 2 formed in a substrate 1 by Ti diffusion.

[0072] Sixth Embodiment 7 is a schematic diagram showing a cross section of an optical modulator 100 according to a sixth embodiment. The optical modulator 100 of this embodiment is a modification of the optical modulator 100 of the third embodiment.

[0073] Referring to FIG. 7 , the optical waveguide 2 is ridge-shaped. In this embodiment, the first electrode 3 has a portion that overlaps with the optical waveguide 2 when viewed in the vertical direction. Furthermore, the first electrode 3 has a portion that overlaps with the optical waveguide 2 when viewed in the horizontal direction. The first electrode 3 may overlap the optical waveguide 2 in the width direction within a range of 10% of the total width W from the widthwise end. The first electrode 3 may overlap the optical waveguide 2 in the thickness direction within a range of 10% of the total thickness T from the upper end. Similar to the first electrode 3, the second electrode 4 has a portion that very slightly overlaps with the optical waveguide 2 when viewed in the vertical direction. Furthermore, the second electrode 4 has a portion that overlaps with the optical waveguide 2 when viewed in the horizontal direction.

[0074] The optical modulator 100 of this embodiment, like the third embodiment, has the same effects as the first embodiment.

[0075] Seventh Embodiment 8 is a schematic diagram showing a cross section of an optical modulator 100 according to a seventh embodiment. The optical modulator 100 of this embodiment is a modification of the optical modulator 100 of the third embodiment.

[0076] 8, in this embodiment, the low dielectric layer 5 and the auxiliary low dielectric layer 6 are integrated. That is, in a cross-sectional view of the optical modulator 100, the entire periphery of the optical waveguide 2 is covered with the integrated low dielectric layer 5 and auxiliary low dielectric layer 6. This increases the electric field passing through the low dielectric layer 5, making it easier to adjust the effective refractive index.

[0077] 8, the first electrode 3 is entirely buried in the low dielectric layer 5. The first electrode 3 may be partially buried in the low dielectric layer 5.

[0078] Eighth Embodiment 9 is a schematic diagram showing a cross section of an optical modulator 101 according to the eighth embodiment. The optical modulator 101 of this embodiment constitutes a Mach-Zehnder type optical modulator. The optical modulator 101 of this embodiment is a modification of the optical modulator 100 of the third embodiment to which the configuration of the second embodiment is applied, and is configured by arranging each element of the optical modulator 100 of the third embodiment in parallel.

[0079] Referring to FIG. 9, an optical modulator 101 of this embodiment includes two optical modulator units 100A and 100B.

[0080] One optical modulator unit 100A includes a substrate 1A, an optical waveguide 2A, a first electrode 3A, a second electrode 4A, a low-dielectric-constant layer 5A, and an auxiliary low-dielectric-constant layer 6A. The other optical modulator unit 100B includes a substrate 1B, an optical waveguide 2B, a first electrode 3B, a second electrode 4B, a low-dielectric-constant layer 5B, and an auxiliary low-dielectric-constant layer 6B. The optical modulator units 100A and 100B are supported by a support plate 7.

[0081] The substrates 1A and 1B correspond to the above-mentioned substrate 1. The optical waveguides 2A and 2B correspond to the above-mentioned optical waveguide 2. The low-dielectric-constant layers 5A and 5B correspond to the above-mentioned low-dielectric-constant layer 5. The first electrodes 3A and 3B correspond to the above-mentioned first electrode 3. The second electrodes 4A and 4B correspond to the above-mentioned second electrode 4. The auxiliary low-dielectric-constant layers 6A and 6B correspond to the above-mentioned auxiliary low-dielectric-constant layer 6.

[0082] Substrate 1A on which optical waveguide 2A is provided is arranged in parallel with substrate 1B on which optical waveguide 2B is provided. In other words, optical waveguide 2A and optical waveguide 2B are arranged side by side and each of them is a ridge type. Upstream of optical waveguide 2A and optical waveguide 2B, a single input optical waveguide branches into optical waveguide 2A and optical waveguide 2B. Downstream of optical waveguide 2A and optical waveguide 2B, optical waveguide 2A and optical waveguide 2B merge into a single output optical waveguide.

[0083] As shown in FIG. 9, in a cross-sectional view of the optical modulator 101, the optical modulator unit 100A is bilaterally symmetrical to the optical modulator unit 100B. That is, the optical modulator unit 100A is symmetrical to the optical modulator unit 100B in the width direction. Specifically, in the optical modulator unit 100A, the first electrode 3A is offset in the width direction toward the optical modulator unit 100B with respect to the optical waveguide 2A, and the second electrode 4A is offset in the width direction to the opposite side of the optical modulator unit 100B with respect to the optical waveguide 2A. Meanwhile, in the optical modulator unit 100B, the first electrode 3B is offset in the width direction toward the optical modulator unit 100A with respect to the optical waveguide 2B, and the second electrode 4B is offset in the width direction to the opposite side of the optical modulator unit 100A with respect to the optical waveguide 2B. In this case, the first electrodes 3A and 3B are positioned closer to each other in the width direction of the optical waveguides 2A and 2B than the second electrodes 4A and 4B. However, in a cross-sectional view of the optical modulator 101, the optical modulator unit 100A may be asymmetrical with the optical modulator unit 100B.

[0084] The optical modulator 101 of this embodiment can also achieve the same effects as those of the first embodiment. Furthermore, since the optical modulator 101 of this embodiment is a Mach-Zehnder type optical modulator, it is possible to perform intensity modulation in addition to phase modulation. This makes it possible to perform multi-level modulation and increase the transmission capacity.

[0085] The optical modulator 101 of this embodiment may not have auxiliary low dielectric constant layers 6A and 6B as in the first embodiment. Also, the optical modulator 101 of this embodiment may not have substrates 1A and 1B as in the seventh embodiment.

[0086] In the optical modulator 101 of this embodiment, the optical waveguides 2A and 2B are ridge-type. Therefore, the same effects as those of the third embodiment can be obtained. However, the optical waveguides 2A and 2B may also be formed by Ti diffusion.

[0087] Ninth Embodiment 10 is a schematic diagram showing a cross section of an optical modulator 101 according to a ninth embodiment. The optical modulator 101 of this embodiment is a modification of the optical modulator 101 of the eighth embodiment.

[0088] 10, the first electrode 3A of the optical modulator unit 100A is formed integrally with the first electrode 3B of the optical modulator unit 100B. That is, the first electrode 3B is electrically integrated with the first electrode 3A. In this case, the first electrode 3B can be shared with the first electrode 3A.

[0089] Tenth Embodiment 11 is a schematic diagram showing a cross section of an optical modulator 101 according to a tenth embodiment. The optical modulator 101 of this embodiment is a modification of the optical modulator 101 of the ninth embodiment.

[0090] Referring to FIG. 11, the substrate 1A of the optical modulator unit 100A is integrated with the substrate 1B of the optical modulator unit 100B. The spontaneous polarization directions of the optical waveguides 2A and 2B are opposite to each other. When the materials of the substrates 1A and 1B are ferroelectric crystals such as LN or LiTaO3, the direction of the spontaneous polarization can be reversed by applying a high voltage to the ferroelectric crystal material. The location of the reversed polarization can be recognized by observation with an atomic force microscope or an electron microscope. In this case, the first electrode 3B can be used in common with the first electrode 3A, and voltages of the same phase are applied to the first electrodes 3A and 3B.

[0091] In the optical modulator 101 of this embodiment, the distance between the optical waveguide 2A and the optical waveguide 2B can be reduced. In this case, the overall width of the optical modulator 101 can be narrowed, and the optical modulator 101 can be made smaller.

[0092] Eleventh Embodiment 12 is a schematic diagram showing a cross section of an optical modulator 100 according to an eleventh embodiment. The optical modulator 100 of this embodiment is a modification of the optical modulator 100 of the third embodiment.

[0093] 12, the first electrode 3 is extended upward. That is, the first electrode 3 extends in the thickness direction from the optical waveguide 2. On the other hand, the second electrode 4 is extended laterally. That is, the second electrode 4 extends in the width direction from the optical waveguide 2. In this case, an electric field can be applied to the low dielectric layer 5 without changing the strength of the electric field.

[0094] However, the first electrode 3 may be extended laterally. That is, the first electrode 3 may extend in the width direction from the optical waveguide 2. On the other hand, the second electrode 4 may be extended downward. That is, the second electrode 4 may extend in the thickness direction from the optical waveguide 2.

[0095] 13A to 13C are schematic diagrams showing the correlation between electrode length and electric field strength. FIGS. 13A to 13C show cross sections of an optical modulator 100. FIG. 13A shows the state when the first electrode 3 and the second electrode 4 are not stretched. FIG. 13B shows the state when only the first electrode 3 is stretched. FIG. 13C shows the state when both the first electrode 3 and the second electrode 4 are stretched. In each of these figures, the electric potential (V) is depicted by contour lines. The narrower the interval between the contour lines, the stronger the electric field (V / m). Outside the optical waveguide 2, the interval between the contour lines narrows in the order of FIG. 13A, FIG. 13B, and FIG. 13C. Therefore, when the first electrode 3 and the second electrode 4 are arranged so as to sandwich the optical waveguide 2 diagonally, if the first electrode 3 is extended upward and the second electrode 4 is extended laterally, as shown in Figure 13C, the electric field relative to the optical waveguide 2 will be stronger.

[0096] Furthermore, the present disclosure is not limited to the above-described embodiments, and various modifications are possible within the scope of the present disclosure. [Explanation of symbols]

[0097] 100, 101: Optical modulator 1: Circuit board 2: Optical waveguide 3: 1st electrode 4:Second electrode 5: Low dielectric constant layer 6: Auxiliary low dielectric constant layer 7: Support plate

Claims

1. an optical waveguide made of a material having an electro-optic effect; a control electrode for controlling light passing through the optical waveguide; a low dielectric constant layer having a dielectric constant lower than that of the optical waveguide; The control electrode includes a first electrode and a second electrode that form a potential difference with each other, In a cross-sectional view perpendicular to the extending direction of the optical waveguide, the first electrode is provided on one side in a width direction of the optical waveguide and on one side in a thickness direction of the optical waveguide, the second electrode is provided on the other side in the width direction of the optical waveguide and on the other side in the thickness direction of the optical waveguide, the low dielectric layer is interposed between the first electrode and the optical waveguide; a portion of the first electrode that is close to the optical waveguide is buried in the low dielectric layer; an optical modulator, wherein the first electrode is disposed on the optical waveguide side and includes a corner portion that is in contact with and embedded in a groove of the low dielectric layer;

2. 2. The optical modulator according to claim 1, In a cross-sectional view perpendicular to the extending direction of the optical waveguide, the optical waveguide includes a first side extending in the width direction and a second side disposed parallel to the first side and extending in the width direction; The optical modulator, wherein the first electrode is provided on the first side.

3. 2. The optical modulator according to claim 1, In a cross-sectional view perpendicular to the extending direction of the optical waveguide, the optical waveguide has a semi-elliptical shape including a base as a major axis extending in the width direction, The optical modulator, wherein the first electrode is provided on the bottom side.

4. The optical modulator according to any one of claims 1 to 3, In a cross-sectional view perpendicular to the extending direction of the optical waveguide, The first electrode has a rectangular shape, an optical modulator, wherein the first electrode is embedded in the surface of the low dielectric layer opposite to the optical waveguide;

5. The optical modulator according to any one of claims 1 to 3, In a cross-sectional view perpendicular to the extending direction of the optical waveguide, the first electrode is provided only on the one side of the center of the optical waveguide in the thickness direction, an optical modulator, wherein the second electrode is provided only on the other side of the center in the thickness direction of the optical waveguide;

6. The optical modulator according to any one of claims 1 to 3, an auxiliary low dielectric layer having a dielectric constant lower than that of the optical waveguide is provided between the second electrode and the optical waveguide;

7. The optical modulator according to any one of claims 1 to 3, The material of the optical waveguide is LiNbO ３ That is, an optical modulator.

8. The optical modulator according to any one of claims 1 to 3, an optical modulator, wherein one of the first electrode and the second electrode extends from the optical waveguide in the thickness direction, and the other extends from the optical waveguide in the width direction.

9. The optical modulator according to any one of claims 1 to 3, further comprising: An optical modulator comprising a substrate on which the optical waveguide is provided.

10. 10. The optical modulator according to claim 9, the substrate is made of the same material as the optical waveguide; An optical modulator, wherein the optical waveguide is a ridge type.

11. 2. The optical modulator according to claim 1, An optical modulator comprising two optical modulator units arranged in parallel, each of which includes the optical waveguide, the control electrode, and the low dielectric constant layer.

12. 12. The optical modulator according to claim 11, An optical modulator, wherein the first electrode of one of the two optical modulator units is formed integrally with the first electrode of the other optical modulator unit.

13. 12. The optical modulator according to claim 11, each of the optical modulator units further includes a substrate on which the optical waveguide is provided; An optical modulator, wherein the substrate of one of the two optical modulator units is arranged in parallel with the substrate of the other optical modulator unit.

14. 14. The optical modulator according to claim 13, In each of the optical modulator units, the substrate is made of the same material as the optical waveguide; An optical modulator, wherein the optical waveguide is a ridge type.

15. 15. An optical modulator according to claim 13 or 14, the substrate of one of the two optical modulator units is integral with the substrate of the other optical modulator unit; the optical waveguide of one optical modulator unit and the optical waveguide of the other optical modulator unit have mutually opposite directions of spontaneous polarization, An optical modulator, wherein voltages of the same phase are applied to the first electrode of the one optical modulator unit and the first electrode of the other optical modulator unit.

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